Conventional methods for outputting paper (recording material) from an image forming apparatus include: front surface paper output (hereinafter referred to as face-up paper output) in which paper is output with a printing face (face where an image is formed) facing up; and back surface paper output (hereinafter referred to as face-down paper output) in which paper is output with a printing face facing down. In the present specification, for convenience, “printing” indicates forming an image on paper.
In a case of an image forming apparatus in which face-up paper output is performed in outputting paper straight, when one-side printing is performed, paper is switched back so that front and back of the paper with respect to a paper carrying direction is inverted and the paper is subjected to the face-down paper output. This is because when a one-side printing job including plural pages is performed in the face-up paper output, the order of pages is inverted. When a two-side printing job including plural pages is performed, an image on a later page faces up in the face-up paper output, and consequently it is unnecessary to switch back the paper.
Paper is switched back by causing the paper to be carried via an inversion carrying route. The inversion carrying route is provided with a pair of inversion rollers that are rotatable forwardly and backwardly, and a carrying direction of paper in the inversion carrying route is inverted by backward rotation (see Patent Document 1 for example).
However, the face-down paper output involving such switchback requires a longer time for the paper to pass through a fixing section and to be output from the image forming apparatus, compared with the face-up paper output in which paper is output straight.
In order to deal with recent speeding up, some of image forming apparatuses in which paper is switched back and output with its face down in one-side printing are designed so that paper having passed through a fixing section is carried at a velocity higher than a process velocity that is a paper carrying velocity of an image forming section, and the paper is output at a velocity higher than a paper output velocity in two-side printing that is the same as the process velocity.
The image forming apparatus may be optionally connected with a post-process apparatus such as a staple apparatus and a sorter apparatus. The image forming apparatus connected with a post-process apparatus constitutes an image forming system (see Patent Document 2 for example).
The post-process apparatus causes paper output from the image forming apparatus to be carried into the post-process apparatus, carries out various post-processes on the paper, and loads the paper on a paper output tray. Examples of the post-process include stapling paper, outputting paper to a specified tray out of plural paper output trays, and offsetting an output position in the paper output tray.
As such an image forming system, Patent Document 3 discloses an arrangement in which a paper carrying velocity in a post-process apparatus changes in accordance with a paper output velocity of an image forming apparatus that changes in accordance with whether face-up paper output or face-down paper output (switchback is required) is carried out.    Patent Document 1: Japanese Unexamined Patent Publication No. Tokukaihei 09-221254 (published on Aug. 26, 1997)    Patent Document 2: Japanese Unexamined Patent Publication No. Tokukai 2005-321482 (published on Nov. 17, 2005)    Patent Document 3: Japanese Unexamined Patent Publication No. Tokukai 2006-124125 (published on May 18, 2006)
However, the Patent Document 3 arrangement in which a paper carrying velocity in the post-process apparatus continuously changes in accordance with a paper output velocity of the image forming apparatus requires a complex control of a velocity of paper carrying means of the post-process apparatus. Consequently, the arrangement requires high costs regardless of whether the arrangement is realized by hardware or software. Further, the arrangement requires a complex control, increasing a risk such as generation of troubles.